Preparation

Melt butter with oil in medium skillet over medium heat. Add mushrooms and sauté until beginning to brown, about 3 minutes. Sprinkle with salt and pepper. Remove from heat. Whisk 1/4 cup Parmesan and next 4 ingredients in small bowl; season with salt and pepper. DO AHEAD: Mushrooms and cheese mixture can be prepared 1 day ahead. Cover separately and chill.

Bake gratin until top is brown and sauce is bubbling at edges, about 1 hour 15 minutes. Let gratin rest 15 minutes before serving. DO AHEAD: Can be made 2 hours ahead. Let stand at room temperature. Tent loosely with foil and rewarm in 300°F oven 20 minutes.

*Available in the produce section of many supermarkets and at specialty foods stores and Italian markets.

**Italian cream cheese; sold at many supermarkets and at Italian markets.